In the field of watchmaking, with the balance, the balance spring forms the time base of mechanical timepieces. The balance spring takes the form broadly of a very fine spring wound into concentric coils wherein a first end, called the first inner coil, is connected to a collet, and a second end, called the last outer coil, is connected to a balance spring stud.
More specifically, the oscillating system comprises a balance/balance spring pair and an escapement. The balance comprises a balance staff connected to a felloe by means of radial arms and pivoted between first and second bearings. The balance spring is attached via a first inner coil to the balance staff, for example, by means of a collet. The balance spring is attached via a last outer coil to an attachment point consisting of a balance spring stud, which may be carried by a stud-holder. The escapement comprises a double roller consisting of a roller that carries an impulse pin and a safety-roller in which is arranged a notch. The escapement also comprises a pallet-lever including a pallet staff pivoted between first and second bearings. The pallet-lever comprises a lever that connects a fork to an entry arm and an exit arm. The fork is formed of an entry horn and an exit horn between which extends a guard pin. The travel of the fork is limited by an entry banking-pin and an exit banking-pin which may be made in one-piece with a pallet-cock. The entry arm and the exit arm respectively carry an entry pallet and an exit pallet. Finally, the pallet-lever cooperates with an escape wheel comprising an escape wheel arbor pivoted between first and second bearings.
The material used to make balance springs is usually an alloy based on cobalt, nickel and chromium. Such an alloy is ductile and must be resistant to corrosion. Recent developments however, propose balance springs made of silicon. Silicon balance springs are more precise than their steel predecessors. However, their cost price is higher. On account of their small dimensions, such balance springs are, however, difficult to assemble.
The balance spring is an Archimedes spring, wound in the horizontal plane, which has only one function: once paired with a balance, it must turn in one direction, and then in the other direction, i.e. oscillate about its position of equilibrium. It is said to “breathe”. Yet, everything conspires to prevent a balance spring from always oscillating at the same frequency. The balance spring must, in particular, be resistant to oxidation and to magnetism which causes the coils to stick to each other and stops the watch. The influence of atmospheric pressure is low. For a long time, temperature was the core of the problem, since heat expands metal and cold causes it to shrink. The balance spring must also be elastic in order to deform and yet always return to its shape.
Above all, the balance spring must be isochronous. Regardless of how far the balance spring turns, it must always take the same time to oscillate. If the balance spring is contracted by only a few degrees, it does not accumulate much energy and returns slowly to its position of equilibrium. If the balance spring is moved far away from its position of equilibrium, it moves very quickly in the opposite direction. What matters is that these two movements take the same amount of time. The underlying idea is that the energy available to the balance spring is not constant and yet despite this, it must operate regardless of whether the watch is completely wound or in its last hours of power reserve.
On account of their small dimensions, such balance springs are, however, difficult to assemble. Yet the manner in which the two ends of the balance spring are attached also has an enormous influence on the accuracy of the timepiece movement. In most mechanical timepiece movements, the two ends of the balance spring are inserted in a pierced element and are immobilised by means of a pin, force-fitted manually using pliers. This may result in a slight rotation of the balance spring, which is detrimental to the accuracy of the rate of the movement. To overcome this problem, in the 1960's, the French watch manufacturer Lip proposed the adhesive bonding of a balance spring with a dot of hot melt adhesive, i.e. an adhesive that is solid at room temperature, but melts under the action of heat.
However, even the technique consisting in bonding the end of balance springs by means of a hot melt adhesive has its limitations. Indeed, it was observed that, because of its viscosity, as it melts, hot melt adhesive exerts a traction force on the balance spring by capillary action and may press the balance spring end against the walls of the stud in which the end is engaged. The resulting deformation of the balance spring induces therein mechanical stresses which are very detrimental to the regularity of its rate.